In Java static initializers can deadlock, on .NET some threads can see uninitialized state in cases where deadlock would occur on the JVM.

JNI

Only supported in the default AppDomain.

Only the JNICALL calling convention is supported! (On Windows, HotSpot appears to also support the cdecl calling convention).

Cannot call string contructors on already existing string instances

A few limitations in Invocation API support

The Invocation API is only supported when running on .NET.

JNI_CreateJavaVM: init options "-verbose[:class|:gc|:jni]", "vfprintf", "exit" and "abort" are not implemented. The JDK 1.1 version of JavaVMInitArgs isn't supported.

JNI_GetDefaultJavaVMInitArgs not implemented

JNI_GetCreatedJavaVMs only returns the JavaVM if the VM was started through JNI or a JNI call that retrieves the JavaVM has already occurred.

DestroyJVM is only partially implemented (it waits until there are no more non-daemon Java threads and then returns JNI_ERR).

DetachCurrentThread doesn't release monitors held by the thread.

Native libraries are never unloaded (because code unloading is not supported).

The JVM allows any reference type to be passed where an interface reference is expected (and to store any reference type in an interface reference type field), on IKVM this results in an IncompatibleClassChangeError.

monitorenter / monitorexit cannot be used on unitialized this reference.

Floating point is not fully spec compliant.

A method returning a boolean that returns an integer other than 0 or 1 behaves differently (this also applies to byte/char/short and for method parameters).

Synchronized blocks are not async exception safe.

Ghost arrays don't throw ArrayStoreException when you store an object that doesn't implement the ghost interface.

Class loading is more eager than on the reference VM.

Interface implementation methods are never really final (interface can be reimplemented by .NET subclasses).

JSR-133 finalization spec change is not fully implemented. The JSR-133 changes dictate that an object should not be finalized unless the Object constructor has run successfully, but this isn't implemented.

Static Compiler (ikvmc)

Some subtle differences with ikvmc compiled code for public members inherited from non-public base classes (so called "access stubs"). Because the access stub lives in a derived class, when accessing a member in a base class, the derived cctor will be run whereas java (and ikvm) only runs the base cctor.

Try blocks around base class ctor invocation result in unverifiable code (no known compilers produce this type of code).

Try/catch blocks before base class ctor invocation result in unverifiable code (this actually happens with the Eclipse compiler when you pass a class literal to the base class ctor and compile with -target 1.4).

Only code compiled in a single assembly fully obeys the JLS binary compatibility rules.

An assembly can only contain one resource with a particular name.

Passing incorrect command line options to ikvmc may result in an exception rather than a proper error messages.

Class Library

Most class library code is based on OpenJDK 6 build 18. Below is a list of divergences and IKVM specific implementation notes.

com.sun.security.auth.module

Not implemented.

java.applet

GNU Classpath implementation. Not implemented.

java.awt

Partial System.Windows.Forms based back-end. Not supported.

java.io.Console

Not implemented.

java.lang.instrument

Not implemented.

java.lang.management

Not implemented.

java.net

No IPv6 support implemented.

java.net.ProxySelector

Getting the default system proxy for a URL is not implemented.

java.text.Bidi

GNU Classpath implementation. Not supported.

java.util.zip

Partially based on GNU Classpath implementation.

javax.imageio.plugins.jpeg

Partial implementation. JPEGs can be read and written, but there is no metadata support.

javax.management

Not implemented.

javax.print

Not implemented.

javax.script

Not implemented.

javax.smartcardio

Not implemented.

javax.sound

Not implemented.

javax.swing

Not supported.

javax.tools

Not implemented.

org.ietfs.jgss

Not implemented.

sun.jdbc.odbc

Implementation based on .NET ODBC managed provider.

sun.net.www.content.audio

Audio content handlers not implemented.

sun.net.www.content.image

Image content handlers not implemented.

The entire public API is available, so "Not implemented." for javax.print, for example, means that the API is there but there is no back-end to provide the actual printing support. "Not supported." means that the code is there and probably works at least somewhat, but that I'm less likely to fix bugs reported in these areas.

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